The present invention relates generally to continuous skin, variable camber airfoils; and, more particularly, to improved actuating mechanisms for such variable camber airfoils which are characterized by their simplicity, rigidity and stability, yet which permit attainment of optimized aerodynamic airfoil configurations and/or reconfigurations without compromising structural wing box design and/or placement, without requiring the continuous flexible skin to function as a load carrying member, without the need for actuating members to project out of the desired optimized aerodynamic contour of the airfoil, and without the need for separate flaps and/or other structures which tend to produce discontinuities in the upper continuous skin surface of the airfoil.
In the design of airfoils--especially in light of today's highly advanced state of aircraft technology--many differing, and sometimes conflicting, design considerations must be taken into account such, merely by way of example, as the operating conditions to which the aircraft is to be subjected--e.g., subsonic, transonic and/or supersonic flight conditions. Each different set of operating conditions presents its own special and, ofttimes, unique problems in terms of desired and/or required airfoil performance characteristics. However, common to the problems of efficient and effective airfoil design is the continuing need to design improved actuating mechanisms for varying camber of the airfoil or, at least, of a portion of the airfoil--i.e., the leading and/or trailing edge(s) of the airfoil--in a smooth, efficient manner so as not to induce flow separation at localized regions on the surface of the airfoil, yet wherein the variable camber portion of the airfoil can be readily deployed to any desired operating position.
Those skilled in the art will appreciate that most conventional airfoil designs in use today--irrespective of whether intended for aircraft having subsonic, transonic and/or supersonic capability, and/or whether the aircraft is intended for commercial or other types of usage--require a rigid structural wing box which generally occupies at least 40% of the chord-wise dimension of the airfoil; such wing box serving to provide structural rigidity for the airfoil, primary structural frame members for attachment of leading edges, trailing edges and/or ailerons, as well as storage space for fuel. However, since trailing edge flaps, ailerons and actuating members therefore also commonly require 40% of the chord-wise dimension of the airfoil, this leaves only on the order of 20%, or less, of the airfoil's chord-wise dimension to accommodate leading edge flaps and actuating mechanisms. These conflicting demands for space have, prior to the advent of the present invention, mandated compromise on the part of the designer--i.e., a specific airfoil design has generally been required for each differing specific operational condition. That is, if, for example, high lift, low speed performance characteristics are required, separate flaps are commonly provided which, unfortunately, inherently result in skin surface discontinuities that deleteriously affect airfoil performance and which require complex actuating mechanisms. Alternatively, if separate flaps cannot be tolerated, the designer is forced to make some other compromise such, for example, as limiting the degree of permissible flap deflection, permitting actuating members to project out of the optimum airfoil contour, and/or permitting the actuating mechanism to penetrate into the critical area desirably reserved for the rigid structural wing box. The present invention obviates the need for such compromises.
From the standpoint of design and/or operating desideratum, a number of points are of primary interest. These include, for example, a variable camber system wherein: (i) the actuating mechanism is compatible with a wide range of airfoil designs and does not denigrate or otherwise compromise such requisite structural considerations as wing box location and or extent; (ii) the actuating mechanism is capable of providing a rigid airfoil structure at each different operating position and wherein load paths are maintained both simple and short; (iii) the variable camber control linkage mechanism is stable at all operating positions and essentially derives no strength from the skin--that is, wherein the airfoil skin does not serve as a load carrying element; (iv) the actuating linkage permits both positive (up) and negative (down) deflection through a maximum range of desired rotational angles while maintaining a smoothly continuous skin surface having a relatively constant curvature consistent with desired aerodynamic contours; and (v), the actuating linkage is equally applicable for usage in either or both of variable camber leading airfoil edges and/or trailing edges.
Many attempts have been made in the past to provide variable camber airfoil surfaces which meet certain selected design requirements and operational parameters. Such prior art attempts have involved many different approaches and have met with varying degrees of success. An early typical approach involved the use of "slip joints" and/or similar overlapping skin surfaces having the ability to "grow" and/or "shrink" as the degree of airfoil camber is increased and decreased. However, the design of such systems has commonly required the elimination of, or significant alteration of, the airfoil wing box. Because rigid wing box designs are eliminated or significantly altered, poor structural load paths result and, often, the skin itself has been required to function as a load carrying element. Moreover, in such systems undesired flow separation is commonly produced at the skin surface discontinuities in the regions of the slip joints or other overlapping flap arrangements. As a result of such limitations, this type of apparatus has generally been limited to usage where the airfoil need only operate in low dynamic pressure regions.
Typical prior art approaches involving "slip joints" or other overlapping flap constructions wherein the skin surface of the airfoil is characterized by one or more surface discontinuities are illustrated in, for example, British Provisional Pat. No. 103,400, Jan. 25, 1917 (a slip joint arrangement and actuating linkage for use in trailing edges); U.S. Pat. No. 1,567,531-Magni (a discontinuous lap joint and actuating linkage for varying camber throughout all or selected portions of the chord-wise dimension of an airfoil); U.S. Pat. No. 1,868,748-Hogan (a discontinuous lap joint and actuating linkage for leading and/or trailing edges); U.S. Pat. No. 3,179,357-Lyon (a slip joint and actuating linkage for trailing edges); and, U.S. Pat. No. 4,012,013-Ball et al (a slip joint and actuating linkage for an inlet ramp on supersonic aircraft). Other types of mechanisms disclosed in the prior art for varying camber along all or a substantial portion of the chord length of the airfoil are disclosed in U.S. Pat. Nos. 1,828,981-Parker, 1,886,362-Antoni, 2,022,806-Grant, and 3,716,209-Pierce.
In general, all of the foregoing proposed constructions result in one or more of (i) flow separation at localized areas of skin surface discontinuities, (ii) elimination or substantial reduction of the structural airfoil wing box with consequent denigration of structural load paths, (iii), lack of rigidity and stability in the actuating mechanism, (iv) a requirement that the skin of the airfoil function as a load carrying element, and/or (v), excessive undesired skin flutter which severely alters the performance characteristics of the airfoil. As a consequence, this type of construction has generally been limited to airfoils used in low dynamic pressure regions.
Another proposed "solution" to the problems inherent with variable camber airfoils has required the use of separate flaps such, for example, as the use of Krueger flaps at an airfoil leading edge in the manner disclosed in U.S. Pat. No. 3,504,870-Cole. Again, this type of construction is characterized by discontinuities in the upper skin surface of the airfoil which characteristically produce flow separation; and, further, is not suitable for cruise camber control but, rather, is limited to usage as a low speed, high lift device of the type commonly employed in take-off and/or landing operational modes.
Efforts to overcome the problems associated with discontinuities in the upper skin surface have contemplated the usage of variable camber leading and/or trailing edges wherein the airfoil employs a flexible skin which is subjected to rather sharp deflection in a highly localized region such, for example, as the arrangement disclosed in U.S. Pat. Nos. 1,763,888-Griswold, II, and 2,749,060-Brady et al. See, also, U.S. Pat. No. 2,650,047-Carhart et al. Because of the relatively sharp skin deflection in a localized region, such attempts have failed to solve the problem of flow separation--i.e., the sharp localized deflection area, although a continuous skin surface, still functions as a significant curvature discontinuity which produces flow separation. Moreover, in such constructions the airfoil skin is subjected to significant stress and, this fact places severe constraints on the degree of deflection permitted.
Other types of variable camber systems employing continuous upper skin surfaces are those disclosed in, for example: U.S. Pat. Nos. 2,763,448-Davie, Jr.; 3,836,099-O'Neill et al; 3,994,451-Cole; 3,994,452-Cole; and 4,053,124-Cole. These types of camber control arrangements commonly employ relatively complex linkages or, linkage mechanisms which project out of the aerodynamic contour of the airfoil. Moreover, they provide only limited camber control, generally do not permit of positive (up) flap control and, commonly violate wing box integrity. As a result of these problems, aircraft employing such control mechanisms are commonly limited to operation in low dynamic pressure regions.
Perhaps the most relevant of the prior art proposals of which the inventor and the inventor's assignee are presently aware are those disclosures in Zapel U.S. Pat. Nos. 4,131,253 and 4,171,787, assigned to the assignee of the present invention, which respectively disclose continuous flexible skin variable camber arrangements for airfoil trailing edge and airfoil leading edges. In these patent disclosures, while the flexible airfoil skin assumes a relatively constant smooth curvature devoid of interruptions throughout the region of airfoil deflection, the arrangements fail to obviate some of the more perplexing problems in variable camber airfoil design. Thus, the upper flexible skin surface is unsupported in all but the position of maximum deflection, thereby resulting in undesired "skin flutter"; only limited deflection is obtained--e.g., in the range of up to 15.degree. to 16.degree. negative deflection for either a leading edge (U.S. Pat. No. 4,171,787) or a trailing edge (U.S. Pat. No. 4,131,253); the actuating linkage mechanism disclosed requires the use of cam grooves and followers which, because of their requisite and inherent "loose fits", produce undesired "flap flutter" or chatter in addition to, and distinct from, the problem of "skin flutter" discussed above; and, such actuating linkages tend to impinge upon wing box integrity, particularly in the regions of the cam rollers, followers and slideways which project into the regions where the leading and trailing spars of a wing box are desirably located. Moreover, the number of individual links required and the arrangements for pivotally interconnecting such links tends to increase the length and complexity of load paths. Because of the foregoing disadvantages, variable camber trailing and/or leading edges of the types disclosed in the aforesaid Zapel patents are generally limited to usage in relatively low speed operating modes.